The occurrence, production, mechanism of biosynthesis, chemistry, mechanisms of action, and potential chemotherapeutic usefulness (as determined in animal tests) of microbial metabolites effective in preventing binding of vitamin B12 (and related factors) by tumor and other cells will be investigated. Tumor cells from tissue culture and from animals will be used in in vitro studies of these binding antagonists derived from various microbial sources. Those found effective in preventing the binding will be tested in mouse tumor systems. The vitamin B12 binding inhibitors will be concentrated from microbial fermentations using an assay based on binding of B12-Co60 by Ehrlich ascites cells from mice and from tissue culture. Differential assays of binding proteins will be run using porcine intrinsic factor response as a control, e.g. only those substances preventing B12 binding by Ehrlich ascites cells will be isolated and those involved in preventing B12 binding by porcine intrinsic factor will be discarded.